The present invention relates to a flow meter, and specifically to a flow meter for measuring a flow rate of a fluid utilizing ultrasonic waves.
Flow meters utilizing ultrasonic waves are in wide use in order to measure flow rates of town gas and LPG (liquefied petroleum gas). Japanese Laid-Open Publication No. 9-189589 discloses a conventional flow meter for measuring a flow rate utilizing ultrasonic waves. FIG. 40 is a longitudinal and vertical cross-sectional view of a conventional flow meter 900, and FIG. 41 is a height direction cross-sectional view of the conventional flow meter 900. FIG. 41 shows a flow path structure of the flow meter 900. FIG. 42 is a cross-sectional view of the flow meter 900 seen in the direction of arrow A shown in FIG. 40. FIG. 42 shows a flow path structure of the flow meter 900 when the flow rate is high. The flow meter 900 includes a flow path wall 105 defining a flow path 101, through which a fluid as a measuring target flows. As shown in FIG. 41, the flow path wall 105 defines a quadrangular flow path cross-section 108 having a pair of longer sides 108A and a pair of shorter sides 108B. A pair of generally quadrangular parallelepiped transceivers 131 for sending and receiving ultrasonic waves propagating across the flow path 101 are provided in the flow path wall 105. One of the transceivers 131 is provided in an upstream part of the flow path wall 105, and the other of the transceivers 131 is provided in a downstream part of the flow path wall 105. Each transceiver 131 has a quadrangular transceiving surface 132 for sending and receiving ultrasonic waves propagating across the flow path 101. The length of the transceiving surface 132 along the shorter sides 108B of the flow path wall 105 is substantially the same as the length of the shorter side 108B of the flow path wall 105. Each transceiver 131 is provided so as to be aligned with the shorter sides 108B.
The flow meter 900 includes a flow rate calculation section 123 for calculating a flow rate of a fluid flowing through the flow path 101 based on a result of the sent and received ultrasonic waves obtained by the pair of transceivers 131. The flow rate calculation section 123 includes a measurement control section 124 connected to each of the pair of transceivers 131, and a calculation section 125 connected to the measurement control section 124.
The flow meter 900 having the above-described structure operates as follows. When a fluid as a measuring target flows through the flow path 101, an ultrasonic wave sent from the upstream transceiver 131 propagates so as to cross the flow path 101 obliquely with respect to a fluid flow direction, and reaches the downstream transceiver 131. An ultrasonic wave sent from the downstream transceiver 131 oppositely propagates so as to cross the flow path 101 obliquely with respect to the fluid flow direction, and reaches the upstream transceiver 131. The measuring control section 124 measures a first propagation time period required for the ultrasonic wave sent from the upstream transceiver 131 to reach the downstream transceiver 131 and a second propagation time period required for the ultrasonic wave sent from the downstream transceiver 131 to reach the upstream transceiver 131. When the fluid flows through the flow path 101, the first propagation time period and the second propagation time period are different from each other. The calculation section 125 calculates the flow rate of the fluid flowing through the flow path 101 based on the first propagation time period and the second propagation time period measured by the measuring control section 124.
When a fluid flows through the flow path 101 at a high flow rate, a high flow rate flow speed distribution R along the flow path cross-section 108 shown in FIG. 42 is obtained. As shown in FIG. 42, the flow rate is substantially uniform along the flow path cross-section 108. When a fluid flows through the flow path 101 at a low flow rate, a low flow rate flow speed distribution S along the flow path cross-section 108 shown in FIG. 40 is obtained. As shown in FIG. 40, the flow rate is lower as it is closer to the flow path wall 105, and the flow rate is maximum at the center. Thus, the flow rate exhibits a parabolic curve distribution. The length of the transceiving surface 132 of each transceiver 131 along the shorter sides 108B of the flow path wall 105 is substantially the same as the length of the shorter side 108B of the flow path wall 105. Each transceiver 131 is provided so as to be aligned with the shorter sides 108B. Therefore, two sides of the surface of each transceiver 131 which receives the ultrasonic wave corresponds to the shorter sides 108B of the flow path 1, and each transceiver 131 receives the ultrasonic wave on the entirety of this surface. As a result, the high flow rate flow speed distribution R and the low flow rate flow speed distribution S can entirely be measured.
However, when the fluid flows through the flow path 101 at a higher flow rate as a result of the measurable flow rate range is enlarged, the flow path cross-section 108 needs to be enlarged. The transceiving surface 132 of each transceiver 131 also needs to be enlarged. This requires production of transceivers 131 having a larger transceiving surface 132, which raises the cost.
When the length of the transceiving surface 132 along the shorter sides 108B of the flow path cross-section 108 is smaller the length of the shorter sides 108B, the flow speed of the entirety of the low flow rate flow speed distribution S cannot be measured. In order to obtain a true flow rate measurement (average flow rate) based on the low flow rate flow speed distribution S, the flow rate of the fluid calculated based on the first propagation time period and the second propagation time period needs to be corrected based on a correction coefficient in accordance with the flow rate. Nor can the high flow rate flow speed distribution R entirely be measured. In order to obtain an average flow rate, the calculated flow rate needs to be corrected based on a correction coefficient in accordance with the flow rate. The correction coefficients are significantly different for a high flow rate area and a low flow rate area. The correction coefficient significantly changes in a transition area between the high flow rate area and the low flow rate area. Therefore, in the case where there is even a slight error in the measured value of the flow rate in the transition area, the slight error is magnified by the correction coefficient which significantly changes in the transition area. As a result, the measurement precision of the flow rate in the transition area is deteriorated.
The present invention, for solving this problem, has an objective of providing a flow meter for measuring a wide flow rate range with high precision.
Another objective of the present invention is to provide a flow meter for reducing a change in the correction coefficient in a transition area between a high flow rate area and a low flow rate area.
A flow meter according to the present invention includes a flow path through which a fluid flows; a pair of transceivers for sending and receiving an ultrasonic wave propagating across the flow path; and a flow calculation section for calculating a flow rate of the fluid flowing through the flow path based on a result of the ultrasonic wave being sent and received by the pair of transceivers. The flow path has an equal flow speed area in which the fluid flows at a substantially equal flow speed over an entire flow rate area ranging from a high flow rate area to a low flow rate area. The pair of transceivers send and receive the ultrasonic wave so that the ultrasonic wave propagates in the equal flow speed area. Thus, the above-described objectives are achieved.
The equal flow speed area may be provided at a position deviated from a center of the flow path in a height direction. The pair of transceivers may be each provided at a position deviated from the center of the flow path in the height direction so that the position of each of the pair of transceivers in the height direction substantially matches the position of the equal flow speed area in the height direction.
The flow path may have a cross-section which has a quadrangular shape defined by two shorter sides extending in the height direction and two longer sides extending in a width direction. The pair of transceivers may be respectively provided on the two shorter sides.
The pair of transceivers may send and receive the ultrasonic wave propagating across the flow path in a direction of the two longer sides.
The flow path may have a cross-section which has a quadrangular shape defined by two shorter sides extending in the height direction and two longer sides extending in a width direction. The pair of transceivers may each have a rectangular transceiving surface for sending and receiving the ultrasonic wave. A deviation amount L1 of each of the transceivers from the center of the flow path in the height direction may fulfill the relationship of (Hxe2x88x92W)xc3x970.3xe2x89xa6L1xe2x89xa6(Hxe2x88x92W)xc3x970.7, and may preferably fulfill the relationship of (Hxe2x88x92W)xc3x970.4xe2x89xa6L1xe2x89xa6(Hxe2x88x92W)xc3x970.6, where H is a length of each of the two shorter sides of the cross-section extending in the height direction, and W is a length of the rectangular transceiving surface of each of the transceivers in the height direction.
The flow path may have a cross-section which has a quadrangular shape defined by two shorter sides extending in the height direction and two longer sides extending in a width direction. The ratio of a length of each of the two longer sides may be 1.1 or more and 5 or less with respect to the length of each of the two shorter sides.
The flow path may have a cross-section which has a quadrangular shape defined by two shorter sides extending in the height direction and two longer sides extending in a width direction. The pair of transceivers may each have a rectangular transceiving surface for sending and receiving the ultrasonic wave. A length W of the transceiving surface of each of the transceivers and a length H of each of the two shorter sides of the flow path extending in the height direction may fulfill the relationship of 0.3xc3x97Hxe2x89xa6Wxe2x89xa60.7xc3x97H.
The height direction of the flow path may be a direction in which gravity acts. The pair of transceivers may be deviated in a direction opposite to the direction in which the gravity acts.
The flow meter may further include an asymmetric flow promotion section for deviating the equal slow speed area in the height direction of the flow path, so that the position of each of the transceivers in the height direction of the flow path substantially matches the position of the equal flow speed area in the height direction.
The flow path may include an inlet section upstream with respect to the pair of transceivers, and the asymmetric flow promotion section deviates a measuring flow path with respect to the inlet section.
The asymmetric flow promotion section maybe provided upstream with respect to the pair of transceivers.
The flow path may include an inlet section provided upstream with respect to the pair of transceivers and an outlet section provided downstream with respect to the pair of transceivers. The inlet section and the outlet section may be provided coaxially with or parallel to each other.
The asymmetric flow promotion section may include a curved section provided upstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction.
The asymmetric flow promotion section may include a step provided on a wall portion of the flow path upstream with respect to the pair of transceivers.
The asymmetric flow promotion section may include a different-shape section provided upstream with respect to the pair of transceivers, the different-shape section including an end provided on a wall portion of the flow path and another end provided on another wall portion of the flow path, the ends having different shapes from each other, and the wall portions facing each other across the height direction.
One of the ends of the different-shape section may be stepped, and the other end may be smoothly curved.
The ends may be deviated from each other in a direction in which the fluid flows.
The asymmetric flow promotion section may include a step provided on a wall portion of the flow path upstream with respect to the pair of transceivers, and a different-shape section provided upstream with respect to the pair of transceivers, the different-shape section including an end provided on a wall portion of the flow path and another end provided on another wall portion of the flow path, the ends having different shapes from each other, and the wall portions facing each other across the height direction.
The asymmetric flow promotion section may include a curved section provided upstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction, and a step provided on a wall portion of the flow path upstream with respect to the pair of transceivers.
The asymmetric flow promotion section may include a curved section provided upstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction, a step provided on a wall portion of the flow path upstream with respect to the pair of transceivers, and a different-shape section including an end provided on a wall portion of the flow path and another end provided on another wall portion of the flow path, the ends having different shapes from each other, and the wall portions facing each other across the height direction.
The asymmetric flow promotion section may include a rectifier provided upstream with respect to the pair of transceivers for providing a resistance against a flow, the resistance varying in the height direction of the flow path.
The pair of transceivers may be each provided at a position deviated from the center of the flow path in the height direction.
The pair of transceivers may be each provided at a position deviated from the center of the flow path in the height direction, so that the position of the equal flow speed area deviated in the height direction of the flow path by the asymmetric flow promotion section substantially matches the position of each of the transceivers in the height direction.
The asymmetric flow promotion section may include a curved section provided upstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction. The pair of transceivers may be each provided at a position deviated toward an outer circumferential surface of the curved section.
The pair of transceivers may each have a rectangular transceiving surface for sending and receiving the ultrasonic wave. The transceiving surface may be smaller than a size of the flow path in the height direction.
The fluid may flow through the flow path both in a forward direction from an upstream position to a downstream position and a rearward direction from a downstream position to an upstream position. The asymmetric flow promotion section may include a forward asymmetric flow promotion section for deviating, in the height direction, the equal flow speed area of the fluid flowing in the forward direction, and a rearward asymmetric flow promotion section for deviating, in the height direction, the equal flow speed area of the fluid flowing in the rearward direction.
The forward asymmetric flow promotion section and the rearward asymmetric flow promotion section may deviate the equal flow speed area in an identical direction.
The asymmetric flow promotion section may include an upstream curved section provided upstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction, and a downstream curved section provided downstream with respect to the pair of transceivers for curving the flow path so that the flow path rises in the height direction. The upstream curved section and the downstream curved section are curved in an identical direction.
The asymmetric flow promotion section may include a step provided on a wall portion of the flow path upstream with respect to the pair of transceivers and a step provided on a wall portion of the flow path downstream with respect to the pair of transceivers, and a different-shape section provided upstream with respect to the pair of transceivers and a different-shape section provided downstream with respect to the pair of transceivers, each of the different-shape sections including one end provided on one wall portion of the flow path and another end provided on another wall portion of the flow path, the ends having different shapes from each other, and the wall portions facing each other across the height direction. The wall portions having the steps upstream and downstream with respect to the pair of transceivers may be on the same side as each other. The wall portions having the one ends of the different-shape sections upstream and downstream with respect to the pair of transceivers may be on the same side as each other, and the wall portions having the another ends of the different-shape sections upstream and downstream with respect to the pair of transceivers are on the same side as each other.
The flow path may be defined by a wall having a pair of openings respectively for exposing the pair of transceivers to the flow path. The flow path further may include a pair of open-hole rectifiers respectively provided between the pair of openings and the flow path for reducing an amount of the fluid flowing into the pair of openings and for alleviating a disturbance of the flow of the fluid through the flow path.
A flow deviation restriction section including fine passage openings may be provided upstream with respect to the pair of transceivers.
A flow deviation restriction section including fine passage openings may be provided downstream with respect to the pair of transceivers.
The pair of open-hole rectifiers may each have fine ultrasonic openings. The fine ultrasonic openings in the open-hole rectifier provided upstream with respect to the pair of transceivers and the fine ultrasonic openings in the open-hole rectifier provided downstream with respect to the pair of transceivers may have different opening sizes or shapes from each other.
The fine ultrasonic openings in the open-hole rectifier provided upstream with respect to the pair of transceivers may have a larger size than a size of the fine ultrasonic openings in the open-hole rectifier provided downstream with respect to the pair of transceivers.